Here is an excerpt from Breaking Free: How Forgiveness and A Course in Miracles Can Set You Free, which explains what A Course in Miracles is.

A Course in Miracles was revealed to a woman named Helen Schucman. Helen was a research psychologist and professor at Columbia-Presbyterian Medical Center in New York City. She had been raised Jewish and had searched for God in the Catholic and Baptist religions, but had concluded that there was no God and that she was an atheist. Helen and a colleague at Columbia, Bill Thetford, had a very conflicted relationship, which made working together difficult. Exasperated, Bill finally stated, "There must be another way" to get along. They made a commitment to treat each other with kindness. Shortly thereafter, Helen heard an inner voice tell her, "This is A Course in Miracles, please take notes." She thought she was going crazy and told Bill about it, and he suggested that she go along with it and take the notes. Bill sensed that the notes had something to do with their commitment to get along better. Hence, A Course in Miracles was dictated over seven years, beginning in 1965. Helen took the notes and Bill transcribed them. The Voice of the Course identified itself as Jesus of the Bible, and Helen believed that it was indeed Jesus. However, she had many doubts and internal conflicts about the entire process. The lives of Helen and Bill changed so dramatically, though, that their experience convinced them this was of great value.

Many people will question the authenticity of the voice of Jesus, even as Helen did. How do we know Helen wasn't just crazy? We don't. But how do we know if Moses really heard the voice of God from a burning bush, or if the Apostle Paul, whose letters form the basis of the Christian religion, really saw the light of Jesus on the road to Damascus? We don't. But the power of the teachings speaks for itself, as do those of ACIM. God did not stop communicating with us two thousand years ago. He speaks to us every moment of every day, in every way imaginable. The evaluation of any spiritual teaching should come from whether it enriches people's lives. A Course in Miracles has done just that, radically improving many lives. It has sold two million copies by word of mouth alone since it came out in the seventies. There are several thousand study groups in over fifty countries, and scores of books have been written about it. It is rapidly rising in popularity, as it satisfies the quest for a usable, practical spiritual path that also accelerates enlightenment.

A Course in Miracles has three sections that are all contained within one book. The Text forms the theoretical basis, the Workbook provides daily lessons designed to shift your thinking toward miracles, and the Manual for Teachers clarifies important points. It is the experience of ACIM's teachings that lead you to see the world differently and to open to your true loving and peaceful nature. Therefore, don't believe what I say; test the ideas on your own. As you read this book, you may find that you doubt or disagree with what I am saying. Good! Think for yourself.